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AF 447 Thread No. 8

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AF 447 Thread No. 8

Old 26th Apr 2012, 08:40
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Recovery trajectories

Since there seems to be consensus that the pitch-down should continue until the stall warning stops, here it is:

N.B. The pitch-up rate has been reduced to 1.5 deg/s to respect stall warning.



Last edited by HazelNuts39; 26th Apr 2012 at 19:20. Reason: Stall warning
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Old 26th Apr 2012, 13:50
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Originally Posted by Owain Glyndwr
If he was fighting an overspeed, why would he select TOGA?
The "crazy speed" comment comes well after selecting TOGA if I recall correctly. There seems to be a significant deterioration in joined-up thinking from the crew - and the PF in particular - after taking manual control.

This is why I am and always have been focused on where the training went wrong (and incidentally, why I get very antsy when others claim I'm arguing for a conclusion that begins and ends with pilot error). The consensus seems to be that the correct course of action would have been to cover the controls, but not actually do anything with them (or indeed leave them alone completely) until the aircraft's response to the conditions had been monitored for at least half a minute. It's all very well saying that from the comfort of our keyboards, but on a psychological level it's another thing entirely to have an aircraft suffer a failure in marginal conditions and retain the willpower to not intervene - especially for less-experienced flight crew.

Holding back and seeing how the aircraft copes is a universal thing, by which I mean it's true regardless of whether one is flying a FBW jet with bells and whistles, or something more traditional - because in both cases the aircraft has been trimmed to remain more-or-less stable. The exception to this rule is of course when a flight surface problem has occurred - but such events are rare enough to be a fair way down the mental list of potential failures.
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Old 26th Apr 2012, 14:20
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Hi DozyWannabe,

From Page 88.
"The A/P2 disconnects. The roll angle changes from 0 to 8.4° in 2 seconds whereas the sidestick is at neutral. The pitch attitude is 0°."

I'm sure they would have loved to have waited 30 secs before touching the controls, but with that roll rate, they could have been beyond 30 degs bank within 7 secs. Most pilots would do something before it went beyond 20 degs.
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Old 26th Apr 2012, 14:44
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Hi rudderrudderrat ,

This is why I said "cover the controls". The aircraft was in moderate turbulence, enough to give it a nasty bump, but probably not enough to turn it over. My suspicion is that the bump that hit at roughly the same time as A/P disconnect is the cause of that roll. It's a nasty rate, for sure - but if you're in turbulence, it's rare for it it last long enough to turn it over, or even much past 10-15 degrees of bank.

Now here's where I need your input. Airliners are designed to be stable. In layman's terms they "want" to fly straight and level. In roll direct, I suspect that 8.4 degree bank angle would remain when exiting the turbulent air pocket - am I right in thinking that unless something has happened to the flight surfaces that angle will hold until corrected?

In any case, the deviation from straight-and-level on exiting the bump was a moderate roll with a minor nose-down pitch component. Of course the roll will need decisive correction - but I think (and please correct me if I'm wrong) that "decisive" can still be smooth and gentle in execution. The pitch component was barely half a point nose-down and should have corrected itself via trim.

The question is why the sudden aggressive pitch-up command when it wasn't necessary?

[ In case it wasn't clear, 30 seconds was a theoretical average - of course that'll be different depending on the circumstances, but I think most agree that the sidestick commands were well in excess of what was required, and to me hint at the classic signs of a startle response. ]
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Old 26th Apr 2012, 15:21
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It was not .5 degree ND. More like 3.5. You assume too much re: bank. The pilot's reactions were necessary. BEA, in #3: "The Pitch reached 10 degrees NU and the a/c began to climb." Have you reviewed how long it took to get to 10 degrees NU? Without horizon, a ten degree input (from outset) is only six degrees NU from cruise.

Whatever the lay analysis; here, at a/p loss, was the beginning of upset, can we agree? In that beginning, a change in LAW and PILOT both became too much to smooth over. I venture that upset began whilst a/p was still latched, for the attitude in PITCH and ROLL needed correction, having begun in auto.

What began as respect for the conditions at hand for 447 has morphed into a mere and casual dismissal of the challenge. c'est la vie..... If only to read a respectful approach to the pilots, I long for the final report.
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Old 26th Apr 2012, 15:39
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So back to Alt laws and such

Looked up the "laws" and reversion sequence again, and seems the system continues to use "normal gee" as the pitch command, and roll rate, but not roll angle "protection". That right?

So why have to hold back stick? Further, the 'bus has a pitch attitude correction for the Nz the plane tries to maintain with stick neutral. Otherwise, at just a few hundredths of a gee less than 1 gee the thing would continue to go nose up in order to maintain 1 gee. We saw this in our little jet once pitch was above 10 or so degrees, and it was quite noticeable above 30 deg of pitch. What, 0.87 gee for constant pitch angle?

I understand some folks pointing out a rapid roll change just after UAS could be disturbing, but seems about right for a bit of turbulence.

I can also understand a very slight nose up command if worried about overspeed, but only for a few seconds and then back to whatever pitch was before the UAS.

The CVR comment about crazy speed might be explained, as vertical velocity was extreme but indicated speed was what, 107 or 135 knots? So PF wondering about why such a slow speed if descending so fast. Maybe started to question the altitude readings, too, for first 30 or 40 seconds.

As another contributor has opined, this will be a "landmark" report and I think the recommendations as well.
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Old 26th Apr 2012, 16:24
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Hi DozyWannabe & salute Gums,

On page 112 Lateral parameters, Estimated side slip shows a distinct asymmetry after the AP disengaged. If I'm reading the trace correctly, it shows a Left Hand side slip DA. Since it was in ALT LAW, the the pilot will have to constantly apply aileron in one particular direction (LH in this case), until the side slip was trimmed to neutral.

I don't think the roll was turbulence induced because the effect lasted too long and would be random L/R. I think the aircraft was out of lateral trim (due some fuel imbalance, or asymmetric thrust, or rudder trim applied by the AP just before disconnect.)

All Boeing pilots are familiar with this problem, because we frequently re-trimmed the rudder during the cruise to keep the ailerons neutral. I've never had to do this on the Airbus, because the AP does it automatically for me.

If the FOs had never flown a Boeing, they would possibly not know about the requirement to re-trim the rudder to facilitate manual flying in ALT Law.
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Old 26th Apr 2012, 16:29
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Lyman, I'm referring to the position on the artificial horizon from 0, not the cruise pitch angle. We've gone through this a dozen times or more, but the autopilot did not induce the climb. There was a nose-up trend before the PF took manual control, but it was gradual, minimal and entirely consistent either with the autopilot regaining cruise pitch, or the aircraft attempting to regain cruise pitch via the trim setting.

The abnormal pitch angles that followed were entirely a result of manual sidestick inputs made by the PF.

Gums, our Francophone colleagues have asserted that "crazy" in this sense refers to high speed, not low speed. The PF talks about a sense or feeling of "crazy speed", and he does not refer to the instruments.

Rudderrudderrat - interesting. However I think turbulence had to have been involved even if the rudder trim was slightly off, because there was a pitch component to it. I know that yaw inputs tend to bring the nose down, but not 3.5 degrees off where it was supposed to be, surely? The estimated side slip trace looks to be in excess of what could be expected if the rudder was out of trim in calm air, no?
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Old 26th Apr 2012, 17:29
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Snoop

Originally Posted by gums
In other words, no "deep stall"
nor a free-faller ! Kiss
Originally Posted by gums
But ya gotta realize you are stalled!
Originally Posted by gums
The CVR comment about crazy speed might be explained, as vertical velocity was extreme but indicated speed was what, 107 or 135 knots? So PF wondering about why such a slow speed if descending so fast. Maybe started to question the altitude readings, too, for first 30 or 40 seconds.
At 02:11:50 Vz=15000 Ft/mn = 148 KTS ; CAS=50 KTS

conclusion is easy :
SPEED VECTOR INTENSITY = (148² + 50²)^.5 = 156 KTS

HOW MUCH IS THE STALL SPEED ?!?!

STALL is a problem of AoA, and never a problem of SPEED !

Giraud did fly to speed near of zero in the airflow without to stall...

Sorry ! That is "unloading the wing", balistic flight, that all combat pilots know.

Last edited by roulishollandais; 26th Apr 2012 at 17:32. Reason: bb-code
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Old 26th Apr 2012, 18:10
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Hi DozyWannabe,

On reflection, I think the side slip trace has some error - I can't see why it would peak at -10 so regularly. So ignoring that:

If you look at Aileron Displacement, before AP disconnect the pattern is symmetrical and seems to include all 4 ailerons.

After AP disconnect, the trace only shows the inner ailerons were active. The deflections show far more LH Inner Aileron around> -10 ( = Left inner UP). It indicates that the aircraft had a continuous desire to roll right, counteracted by a more frequent roll Left command.

The tendency to roll could have been solved with rudder trim.
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Old 26th Apr 2012, 18:31
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Thank you Roul and Rat.

I was not looking carefully at the yaw trace on the recorder.

@ Roul

The technique of "unloading" is maybe foreign to many of the "heavy" pilots. No problem. And my reading of the existing procedures of many airline jets at the time emphasized minimum loss of altitude. Funny, but if you get those wings producing lift quickly, you'll lose less altitude. Worry about the gee limits later when recovering from the lower pitch attitude.

That being said, pilots must realize that you are either approaching a stall or actually in one. Looks to this old dinosaur that the 'bus has excellent yaw stability ( or great dampers) and decent roll characteristics even when the wing is completely stalled. So it comes down to the continued nose up command on AF447. Didn't see drastic roll angles or yaw excursions on the traces. You can pull all you want, but all you'll get is the gee commanded position of the elevators and the THS following to help trim.

What's funny to me was that the primitive FBW system I flew actually moved the stabilizers for nose down when we got to our AoA limit, regardless of our gee command. Gee, imagine using AoA sensors regardless of CAS once weight-on-wheels flipped once airborne.

@Doze

Maybe the overspeed control was what the dude was thinking due to the unusual noise from a fully stalled wing. Dunno.
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Old 26th Apr 2012, 18:37
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@rudderrudderrat

Looking at it again, you're right - the estimated sideslip trace shows evidence of partial data recovery, which is not in itself surprising.

The aileron traces are as you say - but prior to autopilot disconnection they are spiky, indicating regular correction. There's no consistency until after autopilot disconnect, when aileron input, which is manual, trends to the left. This does not necessarily prove a right roll condition, however it does show that the PF *believed* he was correcting a tendency to roll right. But because there is no point at which manual input is not being made post-disconnect, it is impossible to say whether that tendency was consistent, or whether it was transitory.

I'm going to need some input from the better-versed here. I know that upon autopilot disconnect, pitch trim remains in auto in Alternate Law. I know that rudder function reverts to a hard-limited yaw-damper mode in Alternate Law. What I need to know is what happens with rudder trim upon autopilot disconnect in Alternate Law - does it hold the last setting prior to disconnect, or does it return to neutral?
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Old 26th Apr 2012, 18:53
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Hi DozyWannabe,
...what happens with rudder trim upon autopilot disconnect in Alternate Law - does it hold the last setting prior to disconnect, or does it return to neutral?
It holds the last setting.
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Old 26th Apr 2012, 19:04
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OK - next question. Is that behaviour covered in type conversion training? Is there a "CHECK RUDDER TRIM" in the autopilot disconnect procedure?

NB. While this is an interesting line of inquiry, it doesn't explain the magnitude of the roll inputs made by the PF to my mind, or their apparent aggressive reversals. The mantra for manual control on the FBW Airbus is more-or-less "observe and correct and observe and correct". The DFDR traces show a lot of attempted correction, but no significant period of observation.
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Old 26th Apr 2012, 22:10
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Dozy, just personal opinion, but she must have been out of trim in yaw.

It kept wanting to turn right and did a 270 turn to the right in increments on its way down. A swept wing bird at high AOA is very sensitive to rudder input. Very insensitive or even contrary to aileron input.

Why out of trim in yaw? Probably a rigging issue. How often does someone actually check this on a flight-probably never, or almost never.
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Old 26th Apr 2012, 22:19
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@Machinbird - If you substitute "must" with "may", I'd be happier about getting on board (if only to not rule anything out). If the rudder was out of trim, then you'd see consistent corrections in the traces prior to A/P disconnect - there aren't any as far as I can see.

The consistency, magnitude and frequency of manual inputs post-disconnect occlude any reasonable conclusion as to what the aircraft would have done if left to it's own devices. If the PF was correcting a constant roll to the right, why the sudden reversals of bank input to the sidestick? Surely a gradual and consistent left bank input would have been enough prior to stall - after which, as you say, ailerons become ineffective.
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Old 26th Apr 2012, 23:56
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If the rudder was out of trim, then you'd see consistent corrections in the traces prior to A/P disconnect - there aren't any as far as I can see.
If the yaw damper function requires an airspeed input to properly set the gain, it may have ceased functioning. This would likely put the BYDU on the line (but I think I remember information that the BYDU was not activated). In any case, the yaw damping function is not identical between Normal law and Alternate law (Yaw Alt). I know that we have discussed this a number of times in some detail, but I no longer remember the details. Getting forgetful in my old age I guess.


If the PF was correcting a constant roll to the right, why the sudden reversals of bank input to the sidestick? Surely a gradual and consistent left bank input would have been enough prior to stall - after which, as you say, ailerons become ineffective.
Dozy, remember the roll PIO theory? If he had come in gently on the controls, we probably would not be reading about this accident. Instead (probably through inexperience in this flight regime) he came in with ~ 2/3 of full left roll displacement and it took over half a minute before he got the wings settled down. In the meantime, the pitch went to unsustainable levels.

Last edited by Machinbird; 27th Apr 2012 at 00:38. Reason: Add Yaw Alt reference
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Old 27th Apr 2012, 00:29
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Since Giraud has been mentioned:
Something perhaps similar to Giraud's Mont Blanc technique, was taught and used at the French National Gliding Centre at Montaigne Noire, close to Revel, some miles to the east of Toulouse in 1950. This involved landing to the south, up a steep slope and with a tail wind. One aimed to stop near the top so that a short winch launch could be used so a further flight could be made, after turning the glider round and into the wind. Most flights were done taking off and landing along the ridge, to east or west.
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Old 27th Apr 2012, 02:05
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@ Machinbird, DozyWannabe;

This Rudder / Yaw Damper problem has been raised in the past. I have been looking carefully at the DFDR traces post stall and find that the heading is changing at a fairly constant rate (clockwise) when the SS is held over to the left for a reasonable period of time. This lead me to looking at the Inner Aileron traces, and I believe that the RH Inner Aileron when down was effectively creating drag, whereas the LH Inner Aileron when up was in the wake vortex and ineffectual. This drag on the right was causing yaw, and the the Rudder/Yaw Damper didn't exactly help, though in the same vein, the Rudder Trim was offset 0.4 degrees to the left which may have helped create the right wing down bias.
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Old 27th Apr 2012, 02:39
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Well, MM43, how about "adverse yaw"? Could have been a player.

When Doze tried the sim he used rudder to correct roll, not aileron. Using aileron at extreme AoA will definitely cause the nose to move the opposite way you are used to at low AoA. Retired will tell you that in the Phantom, guys would lock the stick between their legs and use rudder for roll when at high AoA. The A-7 was not as bad, but we used the same technique.

Of course, using rudder trim could have helped, if not cured, the constant turn we see in the traces.

Let's face it. The crew was not prepared to fly the beast outside the envelope they were trained to do. Maybe a cosmic test pilot would have seem what was happening and done real fine, but the crew was not trained for that, nor probably had no experience in high performance jets that routinely flew at the limits or even exceeded them. So I'll cut them a small amount of slack in that regard.
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